In the fabrication of semiconductor devices, such as silicon wafers, a variety of different semiconductor equipment and/or processing tools are utilized. One of those processing tools is used for polishing thin, flat semiconductor wafers to obtain a planarized surface. A planarized surface is highly desirable on a shadow trench isolation (STI) layer, on an inter-layer dielectric (ILD) or on an inter-metal dielectric (IMD) layer which are frequently used in modem memory devices. The planarization process is important since in order to fabricate the next level circuit, a high resolution lithographic process must be utilized. The accuracy of a high resolution lithographic process can only be obtained when the process is carried out on a substantially flat surface. The planarization process is therefore an important processing step in the fabrication of a semiconductor device.
A global planarization process can be carried out by a technique known as chemical mechanical polishing or CMD. The process has been widely used on ILD or IMD layers in fabricating modem semiconductor devices. A CMP process is performed by using a rotating platen in combination with a pneumatically actuated polishing head. The process is used primarily for polishing the front surface or the device surface of a semiconductor wafer for achieving planarization and for preparation of the next level processing. A wafer is frequently planarized one or more times during a fabrication process in order for the top surface of the wafer to be as flat as possible. A wafer can be polished in a CMP apparatus by being placed on a carrier and pressed face down on a polishing pad covered with a slurry of colloidal silica or aluminum.
A polishing pad used on a rotating platen is typically constructed in two layers overlying a platen with a resilient layer as an outer layer of the pad. The layers are typically made of a polymeric material such as polyurethane and may include a filler for controlling the dimensional stability of the layers. A polishing pad is typically made several times the diameter of a wafer while the wafer is kept off-center on the pad in order to prevent polishing a non-planar surface onto the wafer. The wafer itself is also rotated during the polishing process to prevent polishing a tapered profile onto the wafer surface. The axis of rotation of the wafer and the axis of rotation of the pad are deliberately not collinear, however, the two axes must be parallel. It is known that uniformity in wafer polishing by a CMP process is a function of pressure, velocity and concentration of the slurry used.
A CMP process is frequently used in the planarization of an ILD or IMD layer on a semiconductor device. Such layers are typically formed of a dielectric material. A most popular dielectric material for such usage is silicon oxide. In a process for polishing a dielectric layer, the goal is to remove typography and yet maintain good uniformity across the entire wafer. The amount of the dielectric material removed is normally between about 5000 .ANG. and about 10,000 .ANG.. The uniformity requirement for ILD or IMD polishing is very stringent since non-uniform dielectric films lead to poor lithography and resulting window etching or plug formation difficulties. The CMP process has also been applied to polishing metals, for instance, in tungsten plug formation and in embedded structures. A metal polishing process involves a polishing chemistry that is significantly different than that required for oxide polishing.
The important component needed in a CMP process is an automated rotating polishing platen and a wafer holder, which both exert a pressure on the wafer and rotate the wafer independently of the rotation of the platen. The polishing or the removal of surface layers is accomplished by a polishing slurry consisting mainly of colloidal silica suspended in deionized water or KOH solution. The slurry is frequently fed by an automatic slurry feeding system in order to ensure the uniform wetting of the polishing pad and the proper delivery and recovery of the slurry. For a high volume wafer fabrication process, automated wafer loading/unloading and a cassette handler are also included in a CMP apparatus.
As the name implies, a CMP process executes a microscopic action of polishing by both chemical and mechanical means. While the exact mechanism for material removal of an oxide layer is not known, it is hypothesized that the surface layer of silicon oxide is removed by a series of chemical reactions which involve the formation of hydrogen bonds with the oxide surface of both the wafer and the slurry particles in a hydroxylation reaction; the formation of hydrogen bonds between the wafer and the slurry; the formation of molecular bonds between the wafer and the slurry; and finally, the breaking of the oxide bond with the wafer or the slurry surface when the slurry particle moves away from the wafer surface. It is generally recognized that the CMP polishing process is not a mechanical abrasion process of slurry against a wafer surface.
While the CMP process provides a number of advantages over the traditional mechanical abrasion type polishing process, a serious drawback for the CMP process is the difficulty in end point detection. The CPM process is frequently carried out without a clear signal about when the process is completed by using only empirical polishing rates and timed polish. Since the calculation of polish time required based on empirical polishing rates is frequently inaccurate, the empirical method fails frequently resulting in serious yield drops. Attempts have been made to utilize an end point mechanism including those of capacitive measurements and optical measurements. However, none of these techniques have been proven to be satisfactory in achieving accurate control of the dielectric layer removed.
Another method for achieving end point detection is marketed by the Applied Materials Corporation of Santa Clara, Calif. in a MIRRA.RTM. CMP device. In the MIRRA.RTM. device, a system of in-situ remote monitor (ISRM) is provided to determine end point by the concept of a periodic change of optical interference. In the MIRRA.RTM. device, signals received from a patterned wafer surface are processed by digital filtering algorithms by a PC programmable filter such that an optical interference intensity changes periodically with the thicknesses of removed surface material. For instance, a built-in laser source which is fixed at 6700 .ANG. wavelength is utilized to cause interference at a wafer surface and thus producing a waveform received by a laser detector. The waveform generated by such a technique is shown in FIG. 1.
FIG. 1 illustrates four cycles of a waveform with each cycle corresponds to a removed material layer thickness of approximately 2437 .ANG.. The technique is adequate to detect an end point in a polishing process wherein only a relatively thin layer, for instance, of only 2000 .ANG. is removed. When a large thickness of material such as an IMD oxide layer having a thickness of at least 4000 .ANG. is to be removed, the method frequently produces faulty results since the laser detector cannot distinguish which one of the waveform cycles the end point falls on. The wafer surface can therefore be either over-polished or under-polished by 2400 .ANG. thickness. In other words, it is difficult for an operator to properly set a "window" of the polishing process to accurately control the thickness of the layer to be removed.
It is therefore an object of the present invention to provide a method for determining an end point in a CMP polishing process utilizing an optical interference technique that does not have the drawbacks or shortcomings of the conventional optical interference method.
It is another object of the present invention to provide a method for determining an end point in a CMP polishing process by utilizing a dual wavelength interference technique such that an expanded process window for end point detection is provided.
It is a further object of the present invention to provide a method for determining an end point in a CMP polishing process by utilizing a dual wavelength interference technique wherein two laser diode sources which emit different wavelength emissions are utilized.
It is another further object of the present invention to provide a method for determining an end point in a CMP polishing process by utilizing a dual wavelength interference technique in which laser emissions of two different wavelengths are utilized to produce a composite wavelength interference pattern by an additive effect.
It is still another object of the present invention to provide a method for determining an end point in a CMP polishing process by utilizing a dual wavelength interference technique in which laser emissions of two different wavelengths are utilized such that a waveform producing a process window of at least 4000 .ANG. is generated.
It is yet another object of the present invention to provide a method for terminating an end point in a chemical mechanical polishing process conducted on a semiconductor wafer by utilizing a dual wavelength interference technique and detecting a preset end point in a waveform cycle which has a period of at least 4000 .ANG..
It is still another further object of the present invention to provide a method for terminating a chemical mechanical polishing process conducted on a semiconductor wafer at a preset end point that is suitable for removing material thicknesses of more than 4000 .ANG. thick.
It is yet another further object of the present invention to provide an apparatus for determining an end point in a chemical mechanical polishing which includes a polishing platen equipped with a laser generator capable of generating emissions at two different wavelengths directed toward a sample surface such that a dual wavelength interference pattern is produced for detecting the end point.